1. Field of the Invention
The present invention relates to a power operated screwdriver.
2. Description of the Prior Art
A typical example of the power operated screwdrivers of the type concerned is disclosed in U.S. Pat. No. 4,655,103. The disclosed screwdriver includes a drive shaft on which a gear, an central disc and a clutch disc are mounted with a compression coil spring disposed between the gear and the central disc. The gear has trapezoidal first cams engageable with to trapezoidal second cams formed on one surface of the central disc. The opposite surface of the central disc has first jaws engageable with second jaws on the clutch disc. When the screwdriver completes a screw-driving operation, the gear and the central disc are separated by the force of the compression coil spring, whereas the central disc and the clutch disc are connected together while the first and second jaws are held out of direct engagement with each other by the force of the compression coil spring.
Since the drive shaft is rotatably mounted in a body of the screwdriver and since the gear is slidably mounted on the drive shaft, rotation of the gear causes the drive shaft to rotate together with the gear due to a friction acting between the inside surface of a central hole in the gear and the outside surface of the drive shaft. Furthermore, the compression coil spring acting directly between the gear and the central disc tends to interconnect the gear and the central disc under the resiliency of the compression coil spring. The rotational driving force of the gear is, therefore, transmitted to the drive shaft via the clutch disc with the result that the drive shaft is rotated together with the gear.
Thus, in the conventional screwdriver, the drive shaft rotates during no-load rotation of the gear before it is axially displaced to positively engage the first and second jaws and also the first and second cams for transmitting the rotational driving force of the gear to the drive shaft to perform a screw-driving operation. When screws are to be driven successively one at a time by the conventional screwdriver, the gear is continuously rotated by a drive motor. Consequently, the co-rotation of the drive shaft and the gear during the no-load operation makes it difficult to fit the driver bit into a groove in the head of each screw reliably and stably. The efficiency of screw-driving operation of the conventional screwdriver is considerably low.